i guess the hd market is slowly moving out of the way for ssd.

Hmm, what will prevent Samsung (a real heavyweight) from buying/taking over WD?
Three big players in the HDD business? Doesn't sound good.

Well, consolidation is always worrisome from the consumer's point of view, but that would still leave Seagate, WD, and Samsung as competitors; Fujitsu amd Toshiba were really niche mfrs anyway so really this should just end up as a stronger Hitachi. There would've been more cause for concern a couple of years ago when the Japanese mfrs were the only sources of sub-2" drives, but Samsung and Seagate have stuck their toes in that water and SSDs are going to take over that segment pretty rapidly anyway.

Might make things a little harder for the Japanese consumer, though, since their domestic machines tend to source Japanese components and they will have only one supplier for HDs.

I would hope that 2.5" drives manufactured by this joint venture would retain the performance and reliability of Hitachi's existing drives. Toshiba and Fujitsu drives offer noticeably worse performance and, in my experience, a higher failure rate, than Hitachi's.